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1. Field of the Invention
The invention relates to a network and methods for connecting multiple computer workstations to a color graphics plotter, particularly to a color thermal graphics plotter.
2. Related Art
Micro-computers and workstations have become commonplace in many work environments, especially engineering work environments. Such devices typically include a graphics driver circuit which produces video signals capable of being displayed in full color on a display screen, or monitor. The video signals may be in any of a variety of formats, but typically include a set of analog signals conveying information for producing red (R), green (G) and blue (B) pixels, as well as information for horizontal and vertical video scan synchronization. Also becoming commonplace are color graphics plotters, which allow the user of a workstation to produce a printed copy in full color of information stored in an electronic file or displayed on the workstation screen. One such plotter is the Schlumberger Graphics model 5232 (now Oce Graphics model G5232) color thermal plotter, which is capable of producing a "B" sized p-lot in about three minutes, when the information to be plotted is downloaded to a graphics processor associated with the plotter and appropriate plot parameters are set. Setting the plot parameters normally involves setting switches located on the front panel of the plotter to indicate (1) whether the plot is to be in line mode, such as for engineering line drawings, or in full color mode, such as for photo-like images requiring color rendering, (2) whether the background of the plot is to be reversed from that which appears on the screen, such as when colored images appearing on a black screen background are to be printed on a white background, and (3) whether the plot is to be situated on the page in portrait or landscape orientation. An additional parameter which may be set is plot size (plot scale factor).
For reasons of cost and space-saving, it is also becoming common for multiple users to share a single plotter. To do so in a practical manner, it is necessary for multiple workstations to be connected to the plotter, and for the users to have a means of sending information to be plotted to the plotter from their workstation seats.
FIG. 1 shows a common way in which such connections have heretofore been mad. Workstations 100, 102, 104 and 106 are connected to respective screens 110, 112, 114 and 116 for display of information at the respective user's work areas. Each of the workstations is also connected to a video multiplexer 118, which passes information to be plotted to a video processor 120, which in turn prepares the information for plotting by a connected plotter 122. Plotter 122 includes a front panel 124 having switches for setting certain of the plot parameters mentioned above. Other of the plot parameters may be set from switches (not illustrated) located on a front panel of the video processor 120. In addition, a respective one of remote control switches 130, 132, 134 and 136 is situated adjacent each workstation and connected to the video multiplexer by, for example, a twisted pair of wires. Actuation of a remote switch by a user instructs multiplexer 118 to "acquire" the display on the user's screen, and to pass it to the video processor for plotting by plotter 122.
Each workstation is connected to its respective screen by a group of as many as five coaxial cables, one each for R, G and B information, and up to two for video synchronization information. Depending on the type of workstation, the video synchronization information may either be (1) provided as separate horizontal and vertical synchronization signals on separate cables (five coax system), or (2) provided as a composite horizontal and vertical synchronization signal on a single cable (four coax system), or (3) provided during blanking intervals of the G information on the "green" video signal cable (three coax system).
Connection of a workstation to the video multiplexer for capture of a screen display, as shown in FIG. 1, typically involves tapping into each of the coaxial cables connecting the workstation to the screen, and running a coaxial cable from the workstation to the video multiplexer for each of the R, G and B signals and for possible synchronization signals. In addition, a video loop through-connector (126, 128) may be required if means is not provided on the workstation's screen for switching off a signal termination device in the screen's circuitry. Thus, depending on the video synchronization format of the workstation, connecting a workstation to the video multiplexer for screen display capture has heretofore required that a twisted pair of RS232 cable for the remote pushbutton switch, and from three to five coaxial cables, be run from the workstation to the multiplexer. Additional dedicated cables (not shown in FIG. 1) would have to be run from the workstation's parallel or serial data output ports to the plotter, via a dedicated processor (not necessarily the video processor 120), for transferring from the workstation to the plotter any digital files which are to be plotted without being displayed. Communication networks linking parallel ports can be costly and are generally dependent on the type of host workstations and micro-computers.
For just one or two users working in close proximity to one another and to the plotter, the conventional solution shown in FIG. 1 might be acceptable. But for a larger number of users to share a plotter, the number and bulk of cables becomes unacceptable for a number of reasons. The cost of so much cabling is high, the bulk of the cabling makes it difficult to route, and the large number of cables can be unsightly and even unsafe if not secured to avoid trip hazards.
The arrangement of FIG. 1 has yet another important disadvantage, particularly when the plotter is located at some distance from one or more of the workstations. Before a remote switch is actuated to initiate a plot of the user's screen display, the user must know (1) whether the front panel switches are set for the desired plot parameters, such as line mode, background reversal, and portrait/landscape, and (2) whether the plotter is ready to acquire a screenful of data. Since a single, high-resolution screen display may contain 2 megabytes or more of pixel data, the video processor must have a buffering capability of at least that much data to accommodate a single frame of screen data. To provide sufficient frame buffering memory for more than one or two plots in the video processor, along with the necessary print spooling logic, can make these devices substantially more costly. Even with print spooling capability, the user must verify whether the multiplexer and video processor are ready to acquire a plot, before instructing them to do so. To abort a plot in process, the user must go to the plotter and actuate an abort switch, if one is provided.
The inconvenience and lost user time with such an arrangement is evident, particularly when users may be situated in different work areas within a room or even in different rooms from the plotter. After setting up the screen display to be plotted, the user may need to walk over to the plotter to verify its correct settings, leave a note for others not to change the settings before he has made his plot, go back to his workstation to instruct the multiplexer to acquire the plot, and then return to the plotter area to retrieve his plot and his note.